1. Field of the Invention
The present invention relates to well jar accelerators used in conjunction with well jars for applying a jarring blow to stuck tubing, drill pipe or other stuck objects or tools in drilling and fishing operations in a well.
2. Description of the Prior Art
A well jar accelerator is designed to be used in conjunction with a well jar in a bottom hole assembly of a fishing string in a well. As used herein, fishing string will be used to include the entire string from the ground or surface level down to the fishing tool at the lowermost end of the fishing string. The fishing string is made up of a running string and a bottom hole assembly which is located below the running string. The bottom hole assembly is typically made up of the well jar accelerator located above a concentrated mass which is located above the well jar. A fishing tool is connected to the lower end of the bottom hole assembly.
The well jar is a percussion tool that operates on a mechanical or hydraulic principle and is designed to deliver a heavy hammer blow to an object stuck in the well. Well jars are often designed to permit blows to be delivered in either an upward ("up" well jar) or downward ("down" well jar) direction. Well jars are used to impart a jarring motion to stuck tubing, drill pipe or other stuck objects or tools in drilling and fishing operations for the purpose of freeing them.
Typically, well jars are constructed with inner and outer telescopically arranged cylindrical assemblies movable longitudinally relative to each other. The cylindrical assemblies have spline portions which are always engaged with each other. This enables the well jar to transmit torque in either direction at all times and in any position of the well jar stroke. The well jars have a delay mechanism, either mechanical or hydraulic, that severely restricts the relative longitudinal movement of the inner and outer cylindrical assemblies when an axial load is first applied. This results in the development of tension or compression in the running string, depending upon the particular well jar's construction and direction of relative movement, i.e., upward or downward. As soon as the delay mechanism releases, the relative movement of the cylindrical assemblies is unrestricted and the cylindrical assemblies are free to move suddenly to the limits of their relative movement to produce a jarring blow to free the stuck object. Examples of well jars of this type and the operation thereof may be found in U.S. Pat. Nos. 3,987,858 and 3,955,634.
During conventional up jarring and down jarring operations with well jars, the intensity of the blow struck is a function of, and proportional to, the accelerated rapid movement of the concentrated mass located immediately above the well jar, with the energy derived from the stretch of the entire running string above the "up" well jar or from the applied weight above the "down" well jar. This accelerated movement of the concentrated mass is often considerably diminished by friction of the running string against the wall of the well. In such cases, much of the energy of the running string is not delivered to accelerate the concentrated mass but is lost to friction. Also, at very shallow depths when jarring either up or down, the lack of available stretch or compression in the short running string causes a great loss in the effective acceleration, greatly reducing the intensity of the blow. If the stretch or compression is very small, the well jar may not impact at all.
A fluid-type well jar accelerator is made up in the fishing string above the well jar with a concentrated mass being located between the well jar and the well jar accelerator. The well jar accelerator increases the magnitude of the delivered jarring blow of the well jar. The well jar accelerator functions as a hydraulic fluid spring which stores energy in a fluid chamber when a strain is applied to the running string. This stored energy is in addition to the energy of the "stretched" or "compressed" running string. When the strain is released by the free stroke (jarring) of the well jar, the stored energy in the well jar accelerator is released. The stored energy accelerates the well jar and concentrated mass either upwards or downwards (depending on the jarring operations) until a blow of high impact is struck.
Typically, the well jar accelerator is constructed with inner and outer telescopically arranged cylindrical assemblies movable longitudinally relative to each other and forming between them a liquid chamber confining an operating liquid. The cylindrical assemblies have spline portions which are always engaged with each other. This allows the well jar accelerator to transmit torque in either direction at all times and in any position of the well jar accelerator stroke. Disposed within-the liquid chamber is a piston assembly. Typically, the piston assembly includes seal means to form a seal with the inner cylindrical assembly and a sliding seal with the outer cylindrical assembly on "up" well jar accelerators and vice versa on "down" well jar accelerators.
During jarring operation of the well jar accelerator in combination with the well jar, the well jar accelerator cylindrical assemblies are moved longitudinally relative to each other, causing the piston assembly to compress the liquid in the liquid chamber storing energy. When the well jar delay mechanism releases and the well jar free strokes, the sudden release of stored energy in the well jar accelerator accelerates the concentrated mass between the well jar and the well jar accelerator at tremendous and intensifying velocity. When the well jar reaches its maximum travel, a blow of higher impact is delivered directly to the stuck object due to the additional stored energy of the well jar accelerator. The action is essentially independent of the running string although the stored elastic energy in the running string can also contribute to the intensity of the delivered blow in deep straight wells. In shallow, deviated, or directional wells, the well jar accelerator is often the main, if not only, source of energy for the jarring operation. The well jar accelerator serves to provide the primary acceleration to the concentrated mass, and does not rely on movement of the entire running string. The well jar accelerator directs the impact of the well jar and concentrated mass to the stuck object, where it is most effective and least damaging to the rest of the fishing string, regardless of depth.
Typically, fluid-type well jar accelerators are hydrostatically balanced so that hydrostatic pressure tends to neither open or close the well jar accelerator. Fluid-type well jar accelerators are prone to over-pressure in the liquid chamber due to thermal expansion of the liquid. Presently, well jar accelerators have no thermal expansion compensation mechanism, and as a result, the increased pressure in the liquid chamber of the well jar accelerator reduces the effectiveness of the jarring operation. The pressure from the thermal expansion of the liquid is added to the pressure generated by operating the well jar accelerator. If a pull in excess of the rating of the well jar accelerator is applied, these combined pressures can reach excessive levels and cause a material or seal failure of the well jar accelerator.
It is desirable to have a well jar accelerator including a thermal expansion compensation mechanism for the liquid in the liquid chamber. It is also desirable to have a well jar accelerator which eliminates the possibility of over-pressurization in the liquid chamber so as not to detrimentally affect the jarring operation or jeopardize the integrity of the well jar accelerator. It is further desirable to have a well jar accelerator having an expansion liquid chamber which replenishes lost operating liquid in a main liquid chamber resulting from minor leakage and maintains the appropriate operating volume and liquid pressure in the main liquid chamber. Additionally, it may also be desirable to have a well jar accelerator with a hydrostatic pressure load imbalance to compensate for the weight of the concentrated mass located below the well jar accelerator.